Disorders of breathing during sleep are now known to constitute a major health problem throughout the world. Obstructive sleep apnea is an extremely common disease which manifests itself in variable degrees of severity. The disease develops when muscle tone of the upper airway diminishes during sleep and negative pressures associated with inspiration result in collapse of the upper airway, preventing air movement and resulting in airway obstruction. The sleeping patient inhales more forcibly, thereby, further lowering upper airway pressures and causing further collapse of the upper airway. During this time, substantially no air movement into the chest occurs and the patient becomes progressively more hypoxic and hypercarbic. Both hypoxemia and hypercarbia produce central nervous system stimulation resulting in arousal. Upon arousal, increase in airway muscle tone opens the airway and the patient rapidly inhales and ventilates quickly to correct the abnormal arterial blood gas values. Generally, the arousal is only modest and the patient is not aware of the arousal. Once blood gas parameters have been corrected, the patient begins to sleep more deeply, upper airway tone again diminishes, and the upper airway collapses resulting in sequential and cyclic apneic arousal episodes.
The duration and severity of each apnea is quite variable from patient to patient and with the same patient throughout the night. Indeed, the disease process represents a spectrum of severity from mild snoring, which is associated with incomplete and inconsequential airway obstruction, to severe apneas which can result in fatal hypoxemia.
This disease commonly results in excessive daytime sleepiness and can disrupt cognitive function during the day due to fragmentation of sleep during the night associated with recurrent arousals of which the patient is not aware.
Although this disease commonly affects obese patients, it may occur in patients with any body habitus. Because this disease is so common and because it presents with the subtle and common symptoms of excessive daytime sleepiness, morning headache, and decreasing ability to concentrate during the day, it is critical that an inexpensive technique for accurately diagnosing and treating this disease be developed. Traditionally, this disease has been diagnosed utilizing a complex and expensive multi-channel polysomnogram. This is generally performed in a sleep lab and involves the continuous and simultaneous measurement and recording of an encephalogram, electromyogram, extraoculogram, chest wall plethysmogram, electrocardiogram, measurements of nasal and oral air flow, and pulse oximetry. These, and often other, channels are measured simultaneously throughout the night and these complex recordings are then analyzed to determine the presence or absence of sleep apnea.
The problem with this traditional approach is that such complex sleep testing costs between one-thousand to thirty five hundred dollars. Since sleep apnea is so common, the cost of diagnosing obstructive sleep apnea in every patient having this disease in the United States would exceed Ten Billion Dollars. It is critical that a new, inexpensive technique of accurately diagnosing sleep apnea be developed.
Nocturnal oximetry alone has been used as a screening tool to screen patients with symptoms suggestive of sleep apnea to identify whether or not oxygen desaturations of hemoglobin occur. Microprocessors have been used to summarize nocturnal oximetry recordings and to calculate the percentage of time spent below certain values of oxygen saturation However, oxygen desaturation of hemoglobin can be caused by artifact, hypoventilation, ventilation perfusion mismatching. For these reasons, such desaturations identified on nocturnal oximetry are not specific for sleep apnea and the diagnosis of sleep apnea has generally required expensive formal polysomnography.
The present invention comprises a system and technique for deriving and utilizing the analysis of graphical pulse oximetry-derived waveforms as a function of time to accurately diagnosis sleep apnea with adequate specificity to, in many cases, eliminate the need for expensive formal polysomnography.
It is the purpose of this invention to provide an inexpensive system for the collection and analysis of pulse oximetry values as a function of time during sleep to provide a diagnosis of sleep apnea with a high degree of specificity.
This invention provides a reliable and specific means for the diagnosis of obstructive sleep apnea which can be performed in the patient's home without attendance of technical personnel. It is further the purpose of this invention to provide an inexpensive and accurate means to both screen for and specifically diagnose obstructive sleep apnea by a single overnight recording in the patient's home without the need for multiple connections to different parts of the patient's body. It is further the purpose of this invention to define a technique for diagnosing obstructive sleep apnea utilizing the calculation of the ascending and descending slope ratio of phasic oxygen desaturations measured during sleep.
Specifically, the present invention defines a device for diagnosing sleep apnea, that has the following components. First, a means must determine an oxygen saturation of a patient's blood. This saturation value is coupled to a means for identifying a desaturation event based on the saturation value. The desaturation event is one in which said oxygen saturation falls below a baseline level by a predetermined amount and for a predetermined time. The slope of the event is calculated by means for calculating a slope of said desaturation event representing a rate of change per unit time of fall of oxygen saturation. This slope is used by a means for comparing said calculated slope with a value of slope which is determined in advance to be indicative of sleep apnea, and determination of diagnosis of sleep apnea is made based on said comparing.
The comparing can be done by:
1) comparing with an absolute number which is likely to indicate a sleep apnea, or PA1 2) comparing with other slopes taken at different times.
The identifying means can also identify a resaturation, immediately following said desaturation and coupled with said desaturation, in which the oxygen saturation rises, and wherein the determination can also be based on a slope of said resaturation.
Many other ways of calculating the slope are also disclosed herein.
These and other aspects of the invention will now be described in detail with reference to the accompanying drawings, wherein: